Alkali metal cyanates were initially produced by oxidation of the corresponding cyanide compounds. However, the hydrolysis which cyanates undergo in hot aqueous solution made isolation of pure material difficult, and this method was later replaced by the fusion reaction of urea with an alkali metal carbonate. In this reaction the carbonate and urea are mixed in an approximately 1:2 molar ratio, heated until the evolution of ammonia subsides, and then additional urea is added and the mixture is heated to the fusion point at 550.degree.-600.degree. C. This method requires intimate mixture of the pulverized reactants prior to heating, and grinding of the solidified reaction melt to recover the cyanate product. In addition, the high reaction temperature results in significant losses of urea through sublimation as well as decomposition of the product by conversion of the cyanate to cyanide. The formation of toxic cyanide not only reduces the yield but also severly limits the use of the cyanate.
In an effort to avoid the blending and grinding problems of the fusion process, the reaction has also been conducted in a rotary ball mill. In this process the reactants are rotated and heated at 150.degree.-300.degree. C for lengthy periods. The need to supply heat to a large reactor and to remove gaseous reaction products results in a very elaborate and complicated unit.
A further development in the process of reacting urea and alkali metal carbonate involves preheating a bed of carbonate and then adding urea in gradually deceasing amounts. The reactants are agitated constantly during the course of the reaction, and less than stoichiometric amounts of urea are added to avoid loss by sublimation or the formation of polymeric urea compounds.
All of these methods suffer from a significant limitation, namely the inability to obtain complete reaction of the urea and carbonate to consistently produce cyanate with a purity greater than about 90 percent. Since the reaction requires the contact of a solid (alkali metal carbonate, e.g. sodium carbonate which melts at 851.degree. C) and a liquid (molten urea, melting at 135.degree. C), the rate of reaction and the degree of completion is inherently governed by the ability of the reactants to come in contact. In the current processes the degree of reaction is limited by the porosity of the carbonate particles. The molten urea initially reacts on the surface of the carbonate to form a solid cyanate coating which effectively blocks the subsurface carbonate from contacting urea for further reaction. Urea added at this point cannot react with the "blocked" carbonate and will either sublime from the reactor or decompose to form water insoluble byproducts.
In an attempt to overcome this limitation, many of the known processes specify that the urea and carbonate reactants must be mixed as thoroughly as possible and also must have as small a particle size as possible. However, even if the solids are intimately mixed and crushed as finely as possible, the increase in contact area is limited and the problem of blocked carbonate is merely spread over a larger number of particles, not overcome.